Technology update

May 28, 2012

Graphene ‘quilts’ cool down transistors

High-power gallium nitride transistors, which often suffer from overheating, could be cooled down using graphene-graphite “quilts” – patchwork structures of coating made from carbon. So say researchers at the University of California at Riverside who have shown that the temperature of hot spots in the transistors can be lowered by as much as 20°, something that increases the lifetimes of the devices ten-fold.

Gallium nitride is a semiconductor routinely employed in bright light-emitting diodes – now routinely found in traffic lights and solid-state lighting. The material is particularly useful in high-power, high-frequency communication devices and in laser diodes that emit violet light. GaN transistors, for their part, have been around commercially since 2006 and have found their way into various wireless applications thanks to their high efficiency and high operating voltages. These devices are also attractive for high-power electronics, in the automotive industry, for example.

The market share of GaN electronics is still limited, however, because the material tends to heat up, especially when operating at high powers and voltages. The excess heat means that devices made of GaN perform less well over time and eventually break down. To avoid this problem, heat generated near the channel of a transistor (the active area of the device) has to be removed efficiently and very fast. Although several thermal management solutions, such as flip-chip bonding of the transistors or placing the devices on composite substrates, have been proposed, none have been completely successful.

Enter graphene quilts

Now, a team led by Alexander Balandin has come up with an unusual solution. The researchers say that GaN transistors can be cooled down using quilts made of graphene-graphite placed on top of the devices – the opposite of what you would expect from a normal quilt. The patchwork structures contain multilayers of graphene and graphite, which are excellent thermal conductors and so carry away excess heat.

“We found that few-layer graphene films preserve their excellent thermal properties even when they are only a few nanometres thick – unlike metal or semiconductor films,” Balandin told nanotechweb.org. “This property makes them excellent as lateral heat spreaders and interconnects.”

Using micro-Raman spectroscopy for in situ monitoring, the researchers found that the temperature of hot spots in GaN transistors operating at high power levels of 13 W/mm of device width can be lowered by as much as 20°. “This temperature reduction translates to an increase in the device lifetime by a factor 10, and to an overall improvement in its reliability,” said Balandin.

Graphene flakes could also be used as fillers in thermal interface materials for GaN substrates, he adds. This means that carbon material could not only help lower the temperature of hot spots but also remove unwanted heat from the entire electronic chip.

“This technique for local heat spreading using materials that preserve their thermal properties at the nanometre scale is a new approach for thermal management,” he said.

The UC Riverside group is already well known in the graphene community for its investigations into low-frequency noise in graphene transistors; for developing the first large-area method for quality control in the material; and for demonstrating the first selective gas sensor made from pristine graphene.

The work described in this story is detailed in Nature Communications and the paper can be read for free.